Re: [PATCH v5 04/10] rust: sync: atomic: Add generic atomics

[Boqun Feng]

On Mon, Jun 23, 2025 at 01:54:38PM +0200, Benno Lossin wrote:
> On Mon Jun 23, 2025 at 7:19 AM CEST, Boqun Feng wrote:
> > On Sat, Jun 21, 2025 at 12:32:12PM +0100, Gary Guo wrote:
> >> Note tha the transparent new types restriction on `AllowAtomic` is not
> >> sufficient for this, as I can define
> >> 
> >
> > Nice catch! I do agree we should disallow `MyWeirdI32`, and I also agree
> > that we should put transmutability as safety requirement for
> > `AllowAtomic`. However, I would suggest we still keep
> > `into_repr`/`from_repr`, and require the implementation to make them
> > provide the same results as transmute(), as a correctness precondition
> > (instead of a safety precondition), in other words, you can still write
> > a `MyWeirdI32`, and it won't cause safety issues, but it'll be
> > incorrect.
> 
> Hmm I don't like keeping the function when we add the transmute
> requirement.
> 
> > The reason why I think we should keep `into_repr`/`from_repr` but add
> > a correctness precondition is that they are easily to implement as safe
> > code for basic types, so it'll be better than a transmute() call. Also
> > considering `Atomic<*mut T>`, would transmuting between integers and
> > pointers act the same as expose_provenance() and
> > from_exposed_provenance()?
> 
> Hmmm, this is indeed a problem for pointers. I guess we do need the
> functions...
> 
> But this also prevents us from adding the transmute requirement, as it
> doesn't hold for pointers. Maybe we need to add the requirement that

The requirement is "transumability", which requires any valid binary
representation of `T` must be a valid binary representation of
`T::Repr`, and we need it regardless whether we use `transumate()` or
not in the implementation. Because for the current implementation,
`from_ptr()` and any atomics may read a value from `Atomic::new()` needs
this. Even if we change the implementation to `Opaque<T::Repr>`, we
still need it for `get_mut()`

> `into_repr`/`from_repr` preserve the binary representation?

We need this too, but just maybe not for safety reasons. Besides, the
precondition that we can say `into_repr`/`from_repr` can preserve binary
representation is the transmutability requirement.

Regards,
Boqun

> 
> ---
> Cheers,
> Benno